Aircraft, in particular transport airplanes, generally have to follow an approach axis when they are landing on a runway. This approach axis can notably correspond to an axis of an instrument approach of the ILS (Instrument Landing System) type or to an approach axis of the GLS (GPS Landing System) type. During such approaches, a unit for monitoring the position of the aircraft, in particular a unit of the MMR (Multi Mode Receiver) type, monitors a position deviation between a current position of the aircraft and the approach axis that the aircraft is supposed to follow. This deviation is transmitted to a device for guiding the aircraft, in order to allow guidance of the aircraft along the approach axis. The guidance device can correspond to an automatic pilot of the aircraft which automatically guides the aircraft along the approach axis in such a way as to reduce this deviation. This makes it possible to carry out an automatic landing function. According to a variant, the guidance device can correspond to a flight director which displays guidance information to a pilot of the aircraft.
As shown in FIG. 1, during an approach in side wind V conditions, the aircraft 1 must carry out a “crabbed” approach, that is to say one in which the front part of the aircraft is engaged along the approach axis 10 but the aircraft is oriented according to a non-zero angle θ (when projected onto a horizontal plane) between a longitudinal axis 12 of the aircraft and the approach axis 10. In FIG. 1, the aircraft is shown with such an orientation for two successive times t1 and t2. The engagement of the front part of the aircraft along the approach axis results from the fact that the current position of the aircraft taken into account by the monitoring unit of the MMR type corresponds to the position of a receiving antenna associated with this monitoring unit, this receiving antenna being situated at a point A at the front of the aircraft, for example in a nose radome. At the moment of the landing of the aircraft on the runway, the pilot of the aircraft must straighten the aircraft, during a maneuver called “de-crabbing”, in such a way as to align the longitudinal axis 12 of the aircraft with the runway 5, in view of taxiing the aircraft on the runway. The act of aligning the longitudinal axis of the aircraft with the runway consists of bringing the longitudinal axis of the aircraft into an orientation in which it is parallel with a longitudinal axis of the runway. During the de-crabbing maneuver, the aircraft pivots about a center of rotation, generally close to its center of gravity. This results in a lateral offset Δ of the trajectory of the aircraft with respect to a longitudinal axis 14 centered on the runway, as shown in FIG. 1 for a time t3 after the landing of the aircraft. This lateral offset is added to a lateral offset (not shown in the figure) due to the error in the guidance of the aircraft along the approach axis.
The certification of the automatic landing function requires that the probability of landing the aircraft laterally beside the runway is less than once in 1,000,000 landings. In order to evaluate this probability, it is appropriate to consider the width of a typical runway and to take account of the distance between the main undercarriages of the aircraft in question, in order to define an authorized maximum lateral offset during a landing of the aircraft. This probability simultaneously takes account of the lateral offset due to the guidance error and of the lateral offset due to a possible side wind during the landing of the aircraft. Consequently, this lateral offset due to the wind reduces the authorized maximum lateral offset for determining an authorized lateral offset due to the guidance error of the aircraft. This reduction of the authorized lateral offset due to the guidance error of the aircraft has the effect of strengthening the performance requirements of the device for guiding the aircraft. This results in an increase of the complexity and of the cost of the device. Moreover, aircraft pilots generally prefer that the aircraft should land substantially on the center of the runway.